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<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<body>
  <script>
    const s4 = new Set();
    s4.add('a').add('b');
    const r1 = s4.delete('a');
    console.log(s4.size); // 1
    console.log(r1);      // true
  </script>
</body>
</html>